The purpose of this study was to evaluate pacemaker patients with less than normal ventricular function to determine if they benefit clinically or subclinically from slower nocturnal pacing rates. It was conducted as a randomized, single blind, cross-over trial. Patients were randomized to an initial demand rate of 50 or 70. After two weeks of this regimen, a 24- hour ambulatory ECG monitor was placed to document the paced rhythm and the overnight heart rate. At the end of two weeks and again after four weeks of the assigned pacing rhythm, patients were admitted to the GCRC for an evening (5 PM) and morning (6AM) echo-Doppler studies. These studies were used to assess systolic and diastolic cardiac function by measuring cardiac volumes and systolic and diastolic function parameters (ejection time, isovolumic relaxation time, ejection fraction, cardiac volumes). Subjects were also asked to complete a questionnaire concerning functional status and symptoms over the preceeding time period. At the end of this period, patients were crossed over to the other pacing rate. Except for pacing rate, no other pacing or medication changes were made. In the present study, we found an improvement in ventricular function at the slower pacing rate of 50 compared to a faster rate of 70. The slower nocturnal heart rate appears to be the major difference between the two regimens. There was a trend towards patients with poorer function having greater improvement at the slower rates when compared to patients with normal ventricular function. The improvement in ventricular function was most apparent on the morning study.